1. Field of the Invention
The present invention relates to a tire mold for molding a pneumatic tire with so-called variable pitch arrangements, and in more detail, the present invention relates to a manufacturing method of a piece in the tire mold for molding which is a so-called divided-type mold provided with segments that consist of a plurality of pieces.
2. Description of the Prior Art
Tire molds for molding a pneumatic tire include so-called a divided-type mold that has a plurality of segments consisting of a plurality of pieces. In the case of a tread pattern of a general pneumatic tire that has no variable pitches, in the divided-type mold, by manufacturing one kind of pitch that is the basis of the minimum repetition pattern of a tread in one piece or in a group of a plurality of pieces, and by arranging the number of said one piece or a group of a plurality of pieces required for the repeated pitches, the pattern for the whole mold can be manufactured.
However, regarding the tire mold for molding a pneumatic tire that has a tread pattern with so-called variable pitch arrangements, unlike the case of a general tire, conventionally, only manufacturing one kind of pitch could not manufacture the whole tread pattern with variable pitch arrangements. Here, the term “variable pitch arrangements” refer to the arrangements in which the pitch lengths of a repetition unit are changed. Since peak level noise that corresponds to the pitch length is spread over a plurality of frequency components by such arrangements, the generation of noise that is offensive to the ear can be inhibited. Here, the more the kinds of pitches that have different pitch lengths are present, the greater the effect of spreading frequency becomes.
In the tire mold for molding that molds such variable pitch arrangements, when four kinds of variations are imparted, for example, conventionally, four kinds of basic pitches were prepared and these basic pitches were arranged randomly or regularly. However, increasing the number of the basic pitches had the problem of raising the cost of manufacturing the mold including the designing cost and also, it had the problem of increasing operation time. Therefore, some tire molds for molding have been proposed that have variable pitch arrangements with reduced number of the pitches as much as possible. For example, in patent document 1 (Japanese Patent Laid-Open Publication No. Hei 10-305713), two kinds of segments (A and B) with different ratio (a:b) of circumferential lengths (circumferential lengths a and b) from the basic point of the segment (different from the segment referred to in the present application) to both dividing lines are prepared and a technique of manufacturing the tire mold for molding that have four kinds of the pitches (AA, AB, BA, and BB) by the combinations of the segments is shown.
However, by the method of the patent document 1, the lug grooves and the like that extend slightly to the circumferential direction come to have different slant angles to the circumferential direction by having the segments whose ratios of the length in the tire circumferential direction are different. As a result, there arises a problem that the properties originally required for the lug grooves such as drainage property and the like might be damaged. There arises another problem that two kinds of segments should be designed even by the method of the patent document 1.
Moreover, since four kinds of pitches (AA, AB, BA, and BB) are the ones with different lengths in the tire circumferential direction respectively, when one pitch is made into one piece, a plurality of pieces with different lengths in the tire circumferential direction are arranged in each segment. In order to contain each piece in a segment with a predetermined length in the circumferential direction, it is necessary that each piece is manufactured as designed. In other words, when the sum of the lengths of all the pieces of a segment in the circumferential direction is different from the predetermined length in the circumferential direction of the specific segment, the sum of the lengths of all the pieces of the segment in the circumferential direction cannot be modified only by replacing with one of the randomly selected pieces and such modification accompanies large amount of labor.